1. Field of Invention
The present invention relates to a method for producing an ethylene-vinyl alcohol copolymer resin, an ethylene-vinyl alcohol copolymer resin obtained by this method for production, and a laminate having a layer containing this resin.
2. Description of the Related Art
Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH”) are superior in oxygen barrier properties, transparency, oil resistance, antistatic properties, mechanical strength and the like, and thus have been widely used as various types of wrapping material and the like such as films, sheets, vessels, etc.
These films and the like are usually formed by a melt molding method. Therefore, EVOH is expected to have superior appearance characteristics in melt molding (being enabling molded articles having superior appearances without generation of gels and seeds (dirt under paint), occurrence of coloring such as yellowing, and the like to be obtained), long-run workability (a property capable of obtaining a molded article without change of physical properties such as viscosity, etc., and without occurrence of fish eye, streak, etc., if molded for a long period of time), and the like. In addition, films, sheets and the like are often formed with a multilayered structure having an EVOH layer for the purpose of improving oxygen barrier properties and the like. When such a laminate is to be obtained, a metal salt is often contained in an EVOH resin in order to improve adhesiveness between layers. However, it is known that particularly when a metal salt is included in an EVOH resin, coloring such as yellowing is more likely to be caused, and thus appearance characteristics are deteriorated.
Under such circumstances, in order to improve various characteristics demanded for EVOH, particularly appearance characteristics, a variety of processes such as a process of irradiating EVOH with an ultraviolet ray (see JP-A No. S50-100194) and a process of irradiating EVOH with a microwave (see JP-A No. H11-291245) were proposed.
However, according to these methods for production, coloring such as yellowing is not sufficiently prevented, and they are further disadvantageous in running costs being high, influences on human bodies being significant, difficulty in adjusting the drying time, and the like. Specifically, for example, when EVOH is irradiated with an ultraviolet ray, energy of the ultraviolet ray is so great that significant negative influences on human bodies may be concerned. Also, when, for example, a microwave is irradiated, deterioration occurs even if irradiated for a short time period of time, and is thus disadvantageous in deteriorated long-run workability.